The Problem: A Starter Cable Has Very Little Room for a Poor Connection
The dashboard end naturally gets most of the attention because that is the part you can see and operate. The more critical fitting point is beneath the bonnet, where the inner cable is secured to the starter switch.
The original factory inner cable was only just long enough. If the end became frayed during installation or adjustment, the retaining grub screw could be left trying to grip a less-than-ideal section of cable.
If that connection cannot be held securely, the cable can pull free. The knob may still look perfectly respectable on the dashboard, but it will no longer operate the starter switch from inside the cockpit.
That makes cable length more important than it might first appear. It also explains why simply copying the original inner cable length is not necessarily the most helpful approach when fitting a replacement.